This invention relates to the provision of overlay coatings incorporating thermal barriers on substrates. Such overlay coatings are employed on components which are subjected to high temperature environments, particularly where corrosion and/or erosion is likely to occur; the primary, but not necessarily sole, application of such coatings is to parts of gas turbine engines, particularly gas turbine combustion can ware, stator and rotor blades and guide vanes.
It has been proposed to produce an overlay coating by first spray coating on to a suitably prepared substrate surface a protection layer of MCrAlY (where M is a suitable metal such as nickel or cobalt or nickel and cobalt) using a plasma deposition process, then spray coating an anchoring coat of a metal by a flame deposition process which produces in the deposit substantially coarser particles than those in the protection layer, and then spray coating a thermal barrier of a refractory material by a plasma deposition process.
While this procedure is, in general, satisfactory there are certain aspects of it which are not ideal. For example, the flame spraying of complex and re-entrant surfaces can be difficult and expensive.